Field of the Invention
The invention relates to a method and an apparatus for guiding and transferring a sheet in a printing machine from a first cylinder to a gripper of a second cylinder using a holding element that guides and transfers a sheet.
In sheet-fed printing machines, the sheets to be printed are held by a first cylinder and are subsequently transferred to a second cylinder. The second cylinder transports the sheet onward and transfers the sheet to a further cylinder. In this way, the sheets, to be printed, pass through the sheet-fed printing machine. In the recto printing operation, the sheet is in each case transferred by the leading edge. In the verso printing operation, the sheet is transferred by the trailing edge by a turner device in a turning operation. The transfer of the sheet should be carried out quickly and precisely and any damage to the sheet should be avoided.
Published German Patent Application DE 19822306 A1 discloses a sheet-fed printing machine that uses a suction gripper in order to transfer the sheet from the first cylinder to a gripper of the second cylinder. The suction gripper is fixed to the second cylinder and pivots against the trailing edge of the sheet, which is still held at the leading edge by a gripper belonging to the first cylinder. The suction gripper attracts the sheet by suction, tensions the sheet in the direction of the trailing edge and subsequently pivots it toward a second gripper, which is arranged on the second cylinder. The second gripper grips the sheet at the trailing edge and the suction gripper then releases the sheet. In a further step, the gripper of the first cylinder also releases the leading edge of the sheet, so that the sheet is transported onward by the second gripper and the second cylinder in the turned state. The suction gripper has to be configured on the second cylinder in a mechanically relatively complicated manner, and because of the required installation space for the suction gripper, the second cylinder cannot be designed as a non-contact transfer drum. The actions of transferring and tensioning the sheet by the suction gripper have to be carried out in a relatively short time, so that high printing speeds are problematic.
It is accordingly an object of the invention to provide an apparatus and a method for guiding and transferring a sheet from a first cylinder to an adjacent second cylinder which overcomes the above-mentioned disadvantages of the prior art apparatus and methods of this general type. In particular, it is an object of the invention to provide a better method and a better apparatus for guiding and transferring a sheet while maintaining register in a printing machine.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for guiding and transferring a sheet in a sheet-processing machine from a first cylinder to an immediately adjacent second cylinder. The apparatus includes: a holding element that guides and transfers the sheet from the first cylinder to the immediately adjacent second cylinder; and a guide device for guiding the holding element. The guide device is held outside the first cylinder and outside the second cylinder, and is preferably parallel to the first and second cylinder or other sheet transport devices. In this way, it is not necessary to adapt the guide device to the overall space available on the first or second cylinder. This means that more flexible construction of the guide device and the cylinders is possible.
In accordance with an added feature of the invention, the guide device is preferably a belt which circulates in the form of an endless belt. The embodiment offers the advantage that the guide device is constructed simply and cost-effectively.
In accordance with an additional feature of the invention, guides for the holding element are located on both sides on the machine frame, together with appropriate flexible drive means, for example a toothed belt or a chain. The holding elements are formed by bridge-like parts.
In accordance with another feature of the invention, the guide device and the holding element are designed in such a way that they can be pivoted into and out of a recess in the second cylinder. In this way, transfer of the sheet can be carried out precisely in the range of the gripper, without the gripper of the second cylinder having to be moved too far out of the circumferential wall of the second cylinder.
In accordance with a further feature of the invention, in addition to an appropriate guide, the holding element can be pivoted into and out of the recess in the second cylinder by superimposing a second movement, which, for example, is controlled by a two-element or multi-element crank mechanism. As a result, it can be ensured that the holding element is braked and pivoted out quickly. This design is particularly suitable for implementing a turner device.
In accordance with a further added feature of the invention, a plurality of tapes with holding elements are provided, which are preferably arranged at different angles with respect to the direction of rotation of the first cylinder. In this way, smooth contact between the sheet and the circumferential wall of the first cylinder can be achieved.
In accordance with a further additional feature of the invention, preferably, at least two holding elements circulate. The two holding elements are each assigned to a trailing edge of two sheets following one another. This provides an efficient configuration of a belt with the holding elements. The belt is preferably driven by a motor at a running speed which corresponds to the rotational speed of the first cylinder. In a development of the invention, the running speed of the belt can be varied, so that the belt, for example in order to tension the sheet, is moved at a lower running speed than the rotational speed of the first cylinder. In this way, tensioning of the sheet on the circumferential surface of the first cylinder is made possible.
In accordance with yet an added feature of the invention, the holding elements are led through between the first and second cylinder and led back around the second cylinder.
In accordance with yet an additional feature of the invention, the holding element can have at least two movable elements, so that the holding element can follow a movement path differing from the guide path. The provided control means, such as control rollers/roller levers and control cams, initiate the appropriate movements. In this case, the advantage resides, in particular, in the fact that the holding element has a speed that differs from the drive means, for example, a chain or a toothed belt, so that a sheet that is held by the holding element can be tensioned.
In accordance with yet another feature of the invention, in order to eliminate the need for further movable elements, which would have to be controlled, the holding element is preferably constructed as a suction gripper.
The holding element preferably has a suction device, with which the sheets can be attracted by suction. As a result of using the suction device, gripping the sheet, and if appropriate, lifting it off the cylinder surface, and also tensioning the sheet, is possible.
The holding elements are preferably associated with the sheet trailing edge, but can also hold or guide the sheet leading edge or other regions of the sheet.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of transporting a sheet through a printing machine, that includes holding a leading edge of the sheet and holding a trailing edge of the sheet while transporting the sheet.
A plurality of holding and guiding devices can be used in the entire area of the printing machine parallel to the sheet-carrying cylinders and this permits a sheet to be transported in such a way that the latter is held both at its leading and at its trailing edge.
In accordance with a concomitant feature of the invention, during a tensioning operation, slippage between the holding element and the sheet is possible.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in method and apparatus for guiding and transferring a sheet in a printing machine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.